1. Field of the Invention
The present invention relates generally to a holographic optical information recording/reproducing device, for recording/reproducing information at a high density using an optical recording medium having a hologram medium. Herein, the “recording/reproducing” refers to an apparatus or method capable of carrying out one or both functions for the purpose of the present invention.
2. Related Background Art
A compact disk (CD) has enabled the recording of music data for 74 minutes and the recording of digital data of 640 megabytes (MB) by means of a light source with a wavelength of 780 nm and an objective lens with a numerical aperture of 0.45. A digital versatile disk (DVD) has enabled the recording of moving pictures of MPEG 2 for two hours and fifteen minutes, and the recording of digital data of 4.7 gigabytes (GB) by means of a light source with a wavelength of 650 nm and an objective lens with a numerical aperture of 0.6. Recently, the high-definition moving picture broadcasting using not less than 1000 horizontal scanning lines, which provides high horizontal resolution, has been started, while personal computers have had higher performances. These have caused the expectations for higher-density and larger-capacity optical disk to rise further. Meanwhile, an optical disk system in which a light source with a wavelength of around 400 nm and an objective lens with a numerical aperture of 0.85 are combined has been proposed, whereby a capacity of more than 20 GB on one side is about to be provided.
Thus, the densification of optical disk devices has been achieved by utilizing an optical source with a shorter wavelength and an objective lens with a greater numerical aperture. However, such an approach as above using a shorter wavelength and a lens with a greater numerical aperture is about to reach its limit. More specifically, in a wavelength range of not higher than 400 nm, the wavelength dispersion of a glass material used in a lens increases, and this makes it difficult to control an aberration of the same. In the case where a solid immersion lens technique that has been developed to increase the numerical aperture is used, a lens working distance is very short (about 50 nm), which makes the changing of disks difficult. To overcome these problems and to achieve higher-densification, the holographic recording technique has attracted keen attention.
A typical example is an optical-disk optical system of the shift-multiplexing recording scheme proposed by Psaltis et al., a schematic configuration of which is shown in FIG. 14. Light from a laser light source 1 is split by a half mirror 8 after its beam diameter is expanded by a beam expander 7. One of the beams obtained by division, whose traveling direction is changed by a mirror 10, passes through a spatial light modulator 2, and is focused into a hologram disk 5 by a Fourier-transform lens 3, where it is converted into a signal beam. The other beam is converged by an objective lens 12 to become a reference beam 22, and irradiates the same position on the hologram disk 5 as that irradiated with the signal beam. The hologram disk 5 is composed of two glass substrates and a hologram medium such as photopolymer provided and sealed therebetween, so that interference fringes produced by the signal beam and the reference beam are recorded.
The spatial light modulator 2 is composed of an optical switch array in which optical switches are arrayed planarly, and the respective optical switches are turned ON/OFF independently according to input signals 23 to be recorded. For instance, in the case where a 1024 cells×1024 cells spatial light modulator 2 is used, information of 1 Mbit can be displayed at once. When signal light passes the spatial light modulator 2, the 1Mbit information displayed on the spatial light modulator 2 is converted into a two-dimensional light beam array and recorded as interference fringes in the hologram disk 5. Upon reproduction of the recorded signals, the hologram disk 5 is irradiated with only the reference beam 22, and a reproduction signal beam 21 as diffracted light from the hologram is passed through a Fourier transform lens 4 and received by a photodetector 6, whereby a reproduction signal 24 is detected.
The optical recording system shown in FIG. 14 is characterized in that the hologram medium is thick, approximately 1 mm, and the interference fringes are recorded as a grating with thick interference fringes, that is, a Bragg grating, thereby enabling angle-multiplexing recording. Thus, a large-capacity optical recording system is provided. In the system shown in FIG. 14, the angle multiplexing is provided by shifting a position irradiated with a spherical wave reference beam, instead of an angle of incidence of the reference beam 22. In other words, slight changes in angles of incidence of the reference beam are utilized that are sensed by respective portions of the medium when a recording position is shifted by slightly rotating the hologram disk 5. When the hologram medium has a thickness of 1 mm, the wavelength selectivity specified according to the reproduction signal intensity has a full width at half maximum (FWHM) of 0.014 degree. With the numerical aperture for the reference beam of 0.5 and the hologram size of 2 mmφ, the recording of multiplexed holograms at intervals of approximately 20 μm results in a recording density of 600 Gbit/inch2, which provides a capacity of 730 GB in the case of a 12-cm disk.
A key to providing a high-density optical recording/reproducing system as described above is a compact and stable laser light source. Particularly, because the Bragg grating has wavelength selectivity as well as angle selectivity, it is necessary to control the wavelength of the light source in recording and reproducing operations, and hence, it is impossible to use a semiconductor laser like that used with a normal optical disk. Besides, though a light source with a short wavelength preferably is used from the viewpoint of recording density, green light of an Ar laser that provides high power at relatively lower costs often is used in experiments. Furthermore, recently, a second-harmonic light source of Nd-doped YAG laser, which can be provided in a completely solid form, is used for achieving compactness.
As described above, in the hologram recording utilizing the Bragg grating, recorded diffraction patterns vary according to incidence directions and wavelengths of light. Therefore, in the case where wavelengths upon recording and reproducing operations are different from each other, this causes an increase in cross-talk signals, a decrease in signal light intensities, and the like. Besides, a change in a temperature of the recording medium causes a change in an optical reproduction wavelength, and this also causes an increase in cross-talk signals, a decrease in signal light intensities, and the like.
In the case of the optical disk of FIG. 14, information is reproduced as Bragg diffracted light from recorded interference fringes. In order that reproduction is carried out with a reproduction signal beam having a sufficient light quantity, it is necessary to satisfy the Bragg condition. In other words, the angle of incidence of the reference beam to the medium and the wavelength of the reference beam need to be optimized.
For instance, assuming a system having a hologram medium with a thickness of 1 mm, a light source wavelength of 515 nm, and an interference fringe cycle of 0.5 μm, an allowable range of the Bragg condition with respect to the reference beam wavelength, which is defined with a value of a wavelength at which the diffraction effect is reduced in half, is 515 nm±0.24 nm. In the case of the configuration of FIG. 14, it is necessary to take thermal expansion of the hologram medium into consideration. This is because the thermal expansion of the hologram medium causes a change in the cycle of the recorded interference fringes, which causes a change in the optimal reproduction wavelength that satisfies the Bragg condition.
The following description will depict an example in which a photopolymer, OmniDex 352 produced by Du Pont, is used. Its thermal expansion coefficient is measured to be 7.1×10−5 (Ueda et al., JP5(1993)-165388A), and a change in the optimal wavelength with respect to a temperature change of 25° C. is 0.18%, and 515+0.9 nm in terms of an oscillation wavelength of argon laser. The change in wavelength is more than three times of the tolerance of the foregoing allowable range of the Bragg condition, 515±0.24 nm, and to stably reproduce holograms against a temperature change of 25° C., it is necessary to optimize the wavelength of the reproduction light source according to a temperature change during reproduction.